Fruit juice...and other solids!
Ever thought of fruit juice as a solid? It may surprise you to know that we can show it's more solid than liquid for a great period of time.
Solids, under gentle imposed stresses, deform elastically and bounce back to their original shape on the removal of that stress. The reason for this “bounce-back-ability” is the presence of a continuous network structure capable of storing energy on deformation. Conversely, liquids do not possess such a structure and therefore do not exhibit elasticity.
Elasticity can be measured with an oscillatory rheology test and phase angle is the parameter we use to quantify it. Simply put, an oscillation measurement on a solid will report a phase angle of 0°: purely elastic behaviour. Conversely, a liquid – completely inelastic (or viscous, as we rheologists call it) – results in a phase angle of 90°. This gives us a scale of “solidity” and therefore a measure of the presence of elastic network structure.
Below is the result of an oscillation stress sweep test on mandarin juice - with phase angle plotted as a function of applied stress:
At low stresses we can clearly see a plateau at a phase angle of around 18°, telling us that the juice, under these gentle, virtually at-rest conditions, possesses a very well developed soft-solid structure. In other words, the juice would be more accurately describe as a solid than a liquid! The reason we don’t see this is that the structure is so weak it is completely disrupted at the slightest disturbance, leading to viscous, liquid-like behaviour. The graphic below shows a comparison of the juice alongside a smoothie and a yoghurt:
As you can see, all three show similar “solidity” at lower stresses, the only differences being in the strength of the structure, which in smoothie and yoghurt are strong enough to make the soft-solid condition noticeable to the casual observer.
So how can we use this information?
Structures, even extremely delicate ones such as we see in the fruit juice, contribute to many desirable characteristics in other foods and beverages as well as in pharmaceuticals, cosmetics, paints and many other manufactured products. These characteristics include:
- Suspension and emulsion stability and sedimentation or creaming resistance.
- Texture and appearance
- Pouring, draining and drip control
- Penetration optimization, sagging and slumping resistance.
An ability to measure these often invisible structures, using techniques such as the oscillation method demonstrated here, can therefore provide a powerful weapon in the armoury of a formulator, product developer or quality controller.
So, tell your friends, when fruit juice is in the carton on the shelf it is a solid – well, more solid than liquid anyway!
If you’d like to learn what structure testing can tell you about your products please contact us.